The invention has been developed with particular attention to its possible use within mobile radio communication systems such as the systems known as GSM and IS-95. In any case, the invention can be applied in any context wherein the functional reception diagram applied is similar, directly or substantially, to the diagram shown in FIG. 1.
To illustrate how the invention can be utilized in an existing system, the diagram of a conventional, single antenna, GSM receiver is presented in FIG. 1. In this diagram, a line 11 receives a baseband digital signal. In addition to an actual useful signal component, the signal comprises a training signal component such as a preamble or a so-called "midamble", essentially comprising a string of binary characters or bits x.sub.p (t) which is assumed to be known. The signal received by the antenna (converted in a form suitable for processing in a module 20X. There the antenna signal is, for instance, converted into baseband and subjected to the conditioning operations currently used in the art for demodulation.
The signal on line 10 is split by a demultiplexing block (not shown in the drawing) between two branches 11 and 12, intended to convey the training signal or the information signal, respectively. It should be stated that, as is well known to the person skilled in the art, such splitting does not necessarily correspond to an actual routing over two different physical channels, since it can be effected in a virtual manner by means of different processing operations.
The processing performed by the first branch 11 is aimed at obtaining an estimate of the channel impulse response whereon the transmitted signal has propagated. This estimated response can be obtained by analyzing how the training signal is effected by the channel. The aforesaid estimate is usually performed by the correlation (or the matched filtering) effected in a module indicated by 14. In block 15, the convolution is calculated between the windowed estimate of the channel impulse response and a set of possible transmitted baseband signals S'.sub.p (t) (over one bit period) to obtain the signal estimates x(t). These estimates are fed to a processing module 16 where the signal routed over a branch 12 arrives after a possible filtering effected in filter 17. This filter has a impulse response equal to the windowed ambiguity function (the ambiguity function being, as known, the auto-correlation of the training sequence), i.e. a response given by [X.sub.p (t)*Xp(T-t)] h.sub.w (t), where h.sub.w (t) is a window function. This method has been described by R. Steel, "Mobile Radio Communications", New York 1992, Chapter 6. In processing module 16 a measurement of the "distance" (incremental metrics for one bit interval) between the generated sequences and the actual received data is performed. The incremental metrics calculated in block 16 are fed through line 18 to a Viterbi processor (known in the art), included in block 19, where the new metrics for each state are established, as occurs in GSM transmission systems. The Viterbi processor is followed in cascade arrangement by a differential decoder (equally known in the art) which emits the output data stream. Essentially, the received signal on line 10 is subjected to a processing operation that can ideally be seen as a complimentary and opposite action to the one effected by the transmission channel.
In the processing module 16 this signal is subjected to a processing operation that can ideally be seen as a complementary and opposite action to the one effected by the transmission channel. All with the purpose of generating as an output, on the line indicated as 18, a signal destined to constitute a replica, as faithful as possible, of the transmitted signal, in view of the subsequent decoding. Such decoding can be performed, for instance, by means of a Viterbi decoder 19, as occurs in GSM transmission systems.
From the analysis of the reference diagram in FIG. 1, it is clear that the more articulated is the set of alternations the transmitted signal--and also the training sequence--may have undergone during transmission over the cannel, the more complex and onerous is the set of processing operations performed in elements 14, 15, 16 and 19.
In particular, in mobile radio systems (at least for base stations, but the use of this technique is being extended also to mobile terminals) the use of diversity reception techniques based on the use of a plurality of N receiving antennas has become widespread. The signal received by an array of antennas of this kind in reality comprises N replicas of the same starting signal, which replicas are received by the aforesaid N antennas in a different way (for example due to a different distribution of the echoes, etc.).
The invention exploits this multiplicity of antennas to develop a more robust receiver structure which enhances the communication link quality. The processing of the individual system at the receiving side entails the analysis of a certain number (for example, M) of symbols received successively. A receiver architecture with N antennas should, for the same propagation environment, consider the analysis of N.times.M symbols. Recovering the transmitted signal in case of the multi-input system essentially entails inverting a system matrix (N.times.j, where N is the number of antennas and j the number of time instants (i.e. the number of consecutive snapshots) considered necessary to faithfully reproduce the transmitted signal. Direct inversion of this matrix can, if done without due attention, lead to noise amplification and instability. Moreover, it can be rather onerous in terms of time and hardware required, and it hardly appears practical for real-time processing of the received signals, as is required in case, for example, of voice signals.